The present invention relates to lamp ballasts, and deals more particularly with a ferroresonant transformer ballast for regulating the current of gas discharge lamps.
The current-voltage characteristics of gas discharge lamps, such as mercury vapor lamps, is nonlinear where the voltage is relatively constant over a range of lamp current which makes a voltage source an unsuitable power source. A current source, on the other hand, has a high output impedance which allows the source voltage to follow the lamp voltage. As shown in FIG. 1, a commonly used method to energize a gas discharge lamp 10 is by means of a variable or alternating voltage source V.sub.in and a ballast 12 coupled in series with the lamp 10 in order to limit the current and to bear the voltage difference between the lamp and the voltage source. However, this method leaves the lamp current, and therefore the lamp output power, sensitive to changes in the input voltage and also reduces the input power factor.
Another method to energize a gas discharge lamp is to use a ferroresonant transformer as an alternating voltage source which has additional benefits as the method described with respect to FIG. 1. Ferroresonant technology, in general, is known for voltage regulation. For example, U.S. Pat. No. 3,573,606 to Kakalec, the teaching of which is herein incorporated by reference, teaches a ferroresonant voltage regulator. Ferroresonant transformers maintain a constant output voltage, limit the output current and improve the input power factor. FIG. 2 schematically illustrates a constant voltage ferroresonant transformer 14. The ferroresonant transformer 14 includes an E-shaped piece 16 and an I-shaped piece 18 cooperating to form a core. An input coil 20 is wound around a center leg 22 of the E-shaped piece 16, and a capacitor coil 24 is wound around a secondary core portion of the center leg 22. An output capacitor (not shown) is coupled in series with the capacitor coil 24. A leakage inductance shunt 26, positioned generally at a longitudinal midpoint of the center leg 22, cooperates with an opposing surface of the E-shaped piece 16 to define an air gap 28.
FIG. 3 schematically illustrates an equivalent electrical circuit of the ferroresonant transformer 14 of FIG. 2, where coils 30 represent the input coil, an inductance 32 having reactance X.sub.S represents the leakage inductance, an inductance 34 having reactance X.sub.M represents the saturable inductance of a secondary portion of the core where the capacitor coil 24 is wound, coils 36 represent the capacitor coil, and capacitor 38 having voltage V.sub.C is the output or resonance capacitor. Regulation is achieved as follows: any increase in the capacitor voltage V.sub.C will further saturate whereby the value of X.sub.M is decreased. A decrease in the value of X.sub.M will also decrease the equivalent capacitance, and in turn decrease the resonant gain. Conversely, any decrease in V.sub.C will reduce the degree of saturation of the core whereby the value of X.sub.M is increased. An increase in the value of X.sub.M will also increase the equivalent capacitance, and in turn will increase the resonant gain. The capacitor root-mean-square (RMS) current is virtually constant over a range of input voltage. As shown in FIG. 4, if a lamp 40 is inserted in series with the output capacitor 38, the capacitor current will adequately energize the lamp 40 provided that the open circuit voltage (the voltage level just before the lamp 40 ignites) is high enough to cause the lamp 40 to strike. The lamp current can be varied by changing the capacitive value of the resonant capacitor 38 which is usually accomplished by interchanging capacitors of varying capacitance.
The saturated core of the ferroresonant transformer increases the crest factor (V.sub.peak /V.sub.rms) of the lamp current which shortens the lamp's operating life and makes it difficult for metal additive lamps to remain lit. Low grade steel reduces the magnitude of the peak capacitor current which makes it the preferred choice for laminations in spite of the higher core losses and reduced efficiency. High power lamps require a high voltage across its terminals. Since the output capacitor 40 is in series with the lamp, as shown in FIG. 4, the output capacitor 40 has to be rated for the same voltage as the lamp. High voltage capacitors are more expensive, more difficult to source, and are physically larger than the standard 660 V type.
It is therefore an object of the present invention to provide a ferroresonant ballast that overcomes the disadvantages associated with prior ballasts for regulating the current of gas discharge lamps.